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The term 'shallot' in Croatia denotes three genetically and morphologically different, vegetatively reproduced relatives of the common onion, Allium cepa L., which are mainly traditionally cultivated for consumption and as a spice: A. cepa Aggregatum group, (2n = 2x = 16), A. × proliferum (Moench) Schrad. (2n = 2x = 16) and A. × cornutum Clementi ex Vis. (2n = 3x = 24). This paper reviews the results of studies of their morpho-anatomical characteristics and genetic structure. Although all three taxa were determined as varieties of the common onion, only the shallot A. cepa Aggregatum group (syn. A. ascalonicum L.) belongs to that species. The shallot A. × proliferum represents a hybrid between the two closely related species, A. cepa and A. fistulosum L. The third form of shallot, A. × cornutum is a still incompletely understood triploid hybrid between A. cepa and one or two closely related Allium species, whose identity has not been fully elucidated. In contrast to shallot A. cepa Aggregatum group, which has normal meiosis and produces fertile seed, hybrid shallots A. × proliferum and A. × cornutum are sterile, and reproduce exclusively vegetatively by underground bulbs or bulbils from the inflorescence.
Allium cepa shallot hybrid polyploidy triploid viviparous onion top onion karyotype genome meiosis